The present invention relates to a multilayer structure for reflecting x-ray radiation and an associated method for analyzing the atomic or molecular composition of a sample through x-ray fluorescence spectroscopy.
Multilayer reflectors, or Bragg reflectors, are often utilized for analyzing structures and detecting the absence or presence of particular atomic elements or molecules. This procedure is generally known as x-ray fluorescence spectroscopy. Such a procedure is useful in detecting impurities of minimal amounts present in the sample of interest. For example, x-ray fluorescence spectroscopy is used in the semiconductor industry for detecting impurities in the silicon and germanium wafers that are the foundation of highly-integrated circuits.
In a typical procedure, an x-ray radiation field is guided to a sample, such as a silicon wafer. The impinging radiation induces a fluorescent radiation field, which is incident upon a multilayer or Bragg reflector. The fluorescent radiation field is directed by the multilayer to a measuring or analyzing detector.
The multilayer functions both as a reflective optic and a frequency selector because only fluorescent radiation that satisfies Bragg""s equation is reflected. Bragg""s equation in general is:
nxcex=2d sin xcex8,xe2x80x83xe2x80x83(1) 
where n is an integral number, A is the wavelength of the initial x-ray radiation field, d is the periodicity of the lattice structure of the multilayer, and 2xcex8 is the angle of diffraction.
Bragg""s equation is satisfied naturally for certain types of crystals that have regular lattice structures. However, typical crystals have spacings of a few tenths of a nanometer, and because soft x-rays have wavelengths between 1-10 nanometers, Equation (1) is not satisfied. Consequently, for soft x-ray analyses using Bragg-type reflections, a multilayer reflector is necessary.
A typical multilayer consists of a substrate upon which layers of two different materials are sequentially deposited, forming a period of layers of thickness d. Generally, one of the materials has a high dielectric constant and the other has a low dielectric constant. Upon impinging at that interface between the dielectric constants, approximately 10xe2x88x922 to 10xe2x88x923 of the incident radiation is reflected. Therefore, a multilayer structure having 102 to 103 layers would theoretically reflect nearly all of the incident radiation. Multilayers have the added advantage of customization, meaning that the d-spacing can be tailored to meet Bragg""s equation for different wavelengths of interest. For example, multilayers can be used to determine the boron content of oxygen carrying materials such as borophosphorsilicate, which is routinely used in the semiconductor industry.
Common multilayers consist of molybdenum-boron carbide (Mo/B4C) periods, lanthanum-boron carbide (La/B4C) periods, and lanthanum-boron (La/B) periods. The Mo/B4C multilayers were considered an improvement over the prior art, but they still rendered the detection of boron impurities problematic. Most importantly, the Mo/B4C pairing, although optimized for the detection of boron, also features a significant reflectivity at an energy of E=90 eV, which is an emission line of silicon known as the Si-Lxcex1 line. This latent reflectivity increases the background signal in certain silicon-containing samples, such as silicon wafers. This limitation hinders the utilization of the Mo/B4C pairing in the semiconductor industry.
Further developments in the field led to the innovation of La/B4C and La/B pairings for multilayers. As compared to the Mo/B4C pairing, the lanthanum-based pairings provided significantly higher reflectivity of the boron emission line of interest, the B-Kxcex1 line. Moreover, background noise created from the Si-Lxcex1 line was significantly suppressed.
However, the lanthanum-based pairings possess structural particularities that result in damage to the multilayer during normal procedures. For example, the lanthanum-based pairings are structurally soft, which leads to a tendency to break, crack or deform during shaping and mounting procedures. Additionally, the lanthanum-based pairings have low resistivity to water, which makes the cleaning process delicate, time-consuming, and potentially damaging to the optic.
Accordingly, the present invention consists of a device and method for improving the detection of boron through x-ray analysis. The present invention digresses from the espoused theory of multilayer reflectors and introduces a multilayer structure in which the periodic elements arise in groups of three as opposed to the previous pairings of materials. By implementing a three-part periodicity, the present invention retains all of the advantages of the prior art while simultaneously overcoming its limitations.
In particular, the present invention consists of a multilayer structure having at least one triad of layers where each of the three layers is a predetermined material. One of the materials is from a group including lanthanum, lanthanum oxide, or lanthanum-based alloys. A second material is disposed between the first material and a third material. The second material is from a group including carbon, silicon, boron, boron carbide or silicon carbide. The third material is from a group including boron or boron carbide.
In a second embodiment, a fourth material is added to further strengthen and increase the water resistance of the multilayer structure. The fourth material is selected from a group including silicon, boron, boron carbide or silicon carbide. The fourth material is disposed between the third layer of multilayer period n and the first layer of multilayer period nxe2x88x921, such that throughout n periods, the respective first and third layers are not adjacent.